I don't know if they do, but if they did, it would be because they had no coloured light receptors (commonly known as cones) but I don't think they would have no colour vision, as they need to be able to spot birds in foilage and whatnot and they need colour sight for that.

It is a common misconception that lack of colour vision is attributable to the absence of cones from the retina, but in fact it is not whether the cones are there or not, but how they are wired up and whether the brain is wired for colour. There are cases of humans developing black and white vision following damage to specific parts of the brain that process colour vision. Analysis of the visual world is a very complicated process and the task is broken down by the brain into multiple sub-levels of processing including colour, movement, and even face recognition. Injury to the discrete brain area that interprets one or more of these modalities leads to the loss of that sense.

A case well-publicised by neurologist Oliver Sacks is of a woman who suffered a lesion to the part of the brain involved in the visual processing of movement. Her visual world consisted of a series of snapshots, rather like an old movie played very slowly. For her, even pouring a cup of tea was a challenge because she could not appreciate the level of liquid rising in the cup. Similarly she found crossing the road a life-threatening experience because she couldn't tell how fast vehicles were approaching.

There are other examples of people who have suffered brain injuries (caused by strokes, tumours, surgeons) to very discrete regions of the brain, and developed intriguing abnormalities including inability to name objects (anomia), inability to recognise faces (prosopagnosia) or word blindess. I have myself seen patients who can write but not read !

Chris

"I never forget a face, but in your case I'll make an exception" - Groucho Marx

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I never forget a face, but in your case I'll make an exception - Groucho Marx

Although a human retina contains both rods and cones the two groups of photoreceptors are not often used simultaneously.

Rods contain a photopigment called scotopsin and are not colour selective. They are much more light sensitive than cones which is why they are the predominant photoreceptor used in the dark. To help make the most of the available light the outputs from many rods are merged together to maximise the chances of detecting even very low levels of light. The increased sensitivity that this produces in the retina comes at the obvious cost of reduced acuity because many outputs are being merged. You'll notice that at night, or under very low light conditions you cannot see colours.

So what happens during the day when the ambient light conditions are sufficiently bright to saturate the rods ? Why can we see clearly under these conditions ? The answer is because the rods are hidden by the pigment epithelium - a melanised layer (contains melanin) at the bakc of the eye. This mops up stray light and stops any reaching the rods. Instead vision is subserved by the cones. Cones are less light sensitive than rods, they contain the the pigment phototopsin and are colour selective. The fovea (the region of the retina offering the highest acuity of vision) is composed entirely of cones.

But what about around dusk when the ambient light is getting bad, but we can still see in colour ? An amazing change takes place in the retina whereby the rods switch on and open an electrical connection to an adjacent cone. This allows you to carry on seeing in colour at much lower light levels than the cones would normally tolerate because the rods help to activate them. The cost of course is a reduced acuity and might help to explain why more road accidents occur at that time of the evening.

Chris

"I never forget a face, but in your case I'll make an exception" - Groucho Marx

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I never forget a face, but in your case I'll make an exception - Groucho Marx

Cats are nocturnal predators and hence their visual system is optimised for night time operation, when light levels are very low. Hence, although they can appreciate colours, cats do not have a well-developed colour visual system like that possessed by daytime-functioning animals, including humans.

To admit the maximum possible amount of light, cats' eyes employ a vertical-eliptical pupil which can open very wide at night (admitting 3 times as much light as the round equivalent). This is why it closes to resemble a 'slit' during the day.

Like humans, light is detected in the retina by 2 types of photoreceptors, rods and cones. Cones are larger than rods and capable of discriminating between different coloured lights because they contain different photopigments (rhodopsins) which only respond to light of specific wavelengths. Put very simply, there are populations of cones which are activated by red light, some which are activated by green light and others which are activated by blue light.

Rods are not colour sensitive. They contain the pigment 'scotopsin', which is more light sensitive than the rhodopsin found in the cones, meaning that less light is needed to activate a rod than a cone, and hence rods are more light sensitive.

At night time there is too little available light for cones to work and so, like a person under the same circumstances, most of a cat's low-light vision is achieved using rods alone, meaning that it sees only in shades of black and white.

Because rods are so light sensitive they are less useful during the day. A special layer at the back of the eye covers them up, and the cones are used instead. Cats have a lesser capacity to appreciate colours than humans do but can discriminate colours which differ greatly in spectral content.

A quick question to tease you - "how would you test whether an animal can see a colour or not ?"

Chris

Here are some references if you want to read more widely about this interesting topic :